Medical devices can be implanted in the bodies of patients for various purposes. Some medical devices detect physiologic events and may apply therapy in response to certain events of interest. For example, a cardiac pacemaker can detect a lull in the beating of the patient's heart and apply an electrical pulse to stimulate the heart into beating again. Unfortunately, certain phenomena around the patient can cause the medical device to indicate detection of a physiologic event, when, in fact, the physiologic event has not occurred.
For example, medical pulse generators (PGs), such as implanted defibrillators and cardiac pacemakers may detect signals from a device, such as a cellular phone, that emits electromagnetic energy. This electromagnetic energy may be interpreted by the PG to be a physiologic event, and the PG may respond as if it were valid. Such inappropriate therapy can be uncomfortable or potentially harmful to the patient. Electromagnetic energy from external devices, which is not useful to the device of interest, is referred to as electromagnetic interference (EMI).
Many implanted medical devices include electronic filters to attenuate EMI signals. Such filters typically pass signals in ranges of frequencies that are selected based on analysis and assumptions about the types of physiologic data and noise data that are expected. For example, if the EMI is assumed to be above 10 MHz, a filter may be implemented that removes signals above the 10 MHz frequency. Unfortunately, some EMI falls below 10 MHz and such a filter will not effectively remove the lower frequency EMI. In addition, because EMI can end up in frequency ranges that are relatively close to or identical to frequencies indicative of actual physiologic events of interest, it can be difficult to design filters that are able to consistently filter out only the EMI, without also filtering out legitimate signals indicative of relevant physiologic events.
As society becomes more technologically advanced, the potential for the problems noted above are increasing, rather than decreasing. For example, today many people carry with them devices, such as cell phones and personal digital assistants (PDAs), which can emit EMI at various levels of intensity. In addition, the increasing prevalence of systems such as security scanners that emit EMI, is a concern for patients who live and work around such devices. Such modern devices may increase the occurrence of interaction between such devices and implanted medical devices.